Calculation of equilibration conditions for garnet granulite and garnet websterite nodules in African kimberlite pipes

Pressure/temperature estimates based on different combinations of calibrated mineralogical thermometers and barometers and alternative assumptions concerning the Fe²⁺/(Fe²⁺+Fe³⁺) ratios in the mineral phases are compared for a suite of fourteen nodules. Preferred P/T values have been obtained by simultaneous solution of eq. (12) of Wood (1974) for garnet-orthopyroxene equilibria and the Ellis and Green (1979) equation defining the P,T,X dependence of the Fe²⁺-Mg²⁺ partition coefficient for coexisting garnet and clinopyroxene. However, to obtain realistic results it is first necessary to calculate the Fe³⁺ contents in the minerals-conveniently done on a charge balance basis. The favoured mean P/T estimates of 654±36°C and 10.8±3.1 kbs are compatible with a lower crustal origin for this nodule suite.